Generally speaking, The Iliad is the oldest piece of western literature that has survived into modern times. It is presumed to date from the eighth century BCE, i.e. 750 BCE, and to have been written by the poet, Homer. Modern translations of The Iliad are taken from the oldest complete manuscripts in existence which date from the tenth century CE but there are papyri from the third century BCE that contain portions of the poem. There has been a long-standing dispute as to whether Homer is the sole author of The Iliad and also as to whether the original format of the poem was written or oral. The vocabulary and poetic consistency of the poem leads most researchers to assume that Homer was, in fact, the sole author of the poem but the debate still goes on. The sheer length of the poem (approximately 15,691 lines) suggests that it was written and not oral but there is no definitive answer to that question either. The poem has been divided into twenty four books, presumably for each letter of the Greek alphabet, but this convention was not necessarily a part of the original format. The name, Iliad, comes from one of the earliest names for Troy, i.e. Ilion. The poem describes the final year of the ten year siege of the city of Troy by the Argives, i.e. the mainland Greeks. The story ends abruptly before the final victory of the Argives over the Trojans and, contrary to popular belief, does not mention the famed Wooden Horse, the murder of the Trojan men or the subjugation of the Trojan women.
The Iliad was part of a series of poems called, appropriately, the Epic Cycle. An unknown number of poems were written to complement The Iliad and dealt with the fall of Troy from a variety of perspectives. Most of the Epic Cycle poems are lost and, with the exception of The Odyssey, the surviving poems exist only in fragments.
The sequel to The Iliad, The Odyssey, is also attributed to Homer and in it we are given further details about the fall of Troy and the Wooden Horse. Whereas The Odyssey can be classified as an adventure story, The Iliad is more of a war chronicle. It tells of the assembly of the armies, the ten years of stalemate fighting and, finally, the intervention of the gods and goddesses to bring the war to a bloody conclusion. The bloodshed and brutality are horrific but we are also given a glimpse into the noble minds and spiritual natures of the warriors who, although they are fighting as pawns of the Olympian Immortals, proudly exercise the logic and mores of their age. Pillaging and slavery were common, accepted practices in the ancient Greek world and the warriors spoke proudly of looting cities and abducting young women. These concepts are appalling to most modern people but, even in this day and age, we still see harsh examples of both barbaric practices. The Greeks and Trojans were proud of their ways of life and found no contradiction in the nobility of freedom and the practicality of keeping slaves. They had no qualms about burning a neighboring city and then becoming morally outraged when their city was threatened with the same fate.
The fall of Troy is loosely dated at about 1250 BCE and is the period of Greek history where the last of the half-mortal/half divine heroes walked upon the earth. The most famous hero, Herakles (Hercules), preceded the Trojan War by one generation and the warriors depicted in The Iliad are the last of their kind to live and procreate on the earth. When Troy fell, the Bronze Age was coming to an end and the Iron Age was dawning. The warriors of the Ashen Spear were destined for the Underworld and mortal men and women were left to live their lives on the surface of the earth with the portions of happiness and/or sorrow that Zeus allotted.
I cannot urge you enough to read The Iliad! It is a marvelous piece of art and history that will amaze and delight you in ways that you may not imagine possible. Some portions of the poem are a little tedious (like the second book, which is commonly called the Catalogue of Ships) but the overall effect is one of exaltation. After you have finished The Iliad you will, of course, be compelled to read The Odyssey. Again, you will not be disappointed.
There are several excellent translations of The Iliad and many horrible translations. I suggest the translations of Richmond Lattimore (ISBN 0226469409) or Robert Fitzgerald (ISBN 0385059418). These books can be found at your local library or you can order them from the Book Shop on this site which is linked to Amazon.com.